Do you have a doorknob that despite being unlocked won’t open the door? You checked to make sure the deadbolt wasn’t obstructing the door and pulled really hard, and looked and you can see that the latch is sticking out from the door and into the door frame? You have a broken latch, my friend.
Before you pick up the phone and call me, there is something that you can do. If you are fortunate enough to recognize this problem with the door open, simply take the old doorknob off using a philips screwdriver in most cases. Then take the latch off with the same screwdriver. Go buy a new latch either from a local store or the internet.
If the latch failed and the door is closed, the problem is slightly greater but not insurmountable. If you are locked inside and you would push the door open, try opening it with a library card. If that doesn’t work, try to unscrew the doorknob and remove it so that you can manipulate the latch and try pulling various parts of it to see if that allows you to open the door.
If you are locked outside and you would pull the door out, try bypassing the doorknob latch with a library card or similar. If you can’t open the door with a card, there isn’t a lot you can do besides look for an open window. This assumes there are no other doors that you can use to gain entry and unscrew the doorknob.
If you can’t get the doorknob off then your remaining options are few. You can either try to destroy the doorknob which may result in damage to the door or you can call me. I can say that it is usually inexpensive to repair this problem if the doorknob can be removed. If the door is closed and the problem is a malfunctioning mortise lock or the door is closed and it is the only door, the lock may be damaged and a replacement may be required. Still, the cost for me to come out and drill off a doorknob would be less than $150, replacement included.
Good luck with your broken door knob. Don’t hesitate to call!